Thank you Jexelot. I know all this and I checked the possibility of my itinerary in advance, before I asked FB to do this. It is not question if the routing is possible, it is more about if FB wants to do it. Till now, my feeling is they are not able to do it because of unexperienced staff, because it it too complicated work, or just becuase they do not want to give away such a tickets for miles :-(

I accepted some Y instead of C in the itinerary, it is not a problem. But even that did not help.

Is there any travel agent who can prepare the possible route and check the award availability, so I will just post it to FB for processing?